For the purpose of this description, a "tunnel-lagging element" will be understood to be a structure adapted to be embedded in the concrete lining and forming a stay or support.
It is known in the building of tunnels in accordance with modern techniques, hereinafter referred to as the "Austrian technique," to provide tunnel-lagging elements as U-shaped steel arches which are embedded in injection or spray concrete applied to the tunnel wall as a lining and structurally different material.
These heavy steel arches, which can have a weight per running meter of 21 to, for example, 27 kgs are already in use and must be bent to the desired wall contour of the tunnel. They are customarily supplied in piece lengths of 3 to 4 meters, i.e. as arch segments, and are connected together by fishplates or the like.
The resulting arches, assembled from these rigid segments, are themselves relatively stiff and have a cross section dimensioned to accept the theoretical maximum or bursting pressure, depending upon the direction of the expected stress and customarily are over dimensioned to be able to withstand several times the expected bursting forces.
As a practical matter, the arches cannot be spaced the desired 10 to 20 cm from the surface of the wall of the tunnel as is preferred to enable them to withstand the nominal bursting force, but may lie some 50 to 100 cm therefrom, with the space between the arch and the wall being filled with spray or injection concrete. Hence the concreting of the arch and the wall is a time-consuming and expensive proposition, especially since large gaps must be filled between the surface of the tunnel wall and the stay or lagging element. The concreted structure is generally relatively rigid and, as a rigid structure, must be capable of withstanding the static and dynamic stresses which may result from earth movements, settling and the like. Here again the tunnel lining and the arches must be of sufficient cross section to provide the support forces.
It has already been proposed to provide a thin reinforced spray or injection concrete shell along the tunnel wall as a so-called semistiff tunnel lining. Such linings can permit controlled movements of the tunnel wall without disruption of the shell until the entire system stabilizes. In this case, the reinforcements may be bars embedded in the concrete. The stiff arches mentioned previously are generally unsuitable for this approach to tunnel-wall lining.